The invention is directed to inorganic pigments, especially cobalt containing inorganic pigments having a spinel structure and higher covering power and a process for their production.
A considerable portion of inorganic bright colored pigments are derived from the crystallographic group of spinels. These have the general formula AB.sub.2 O.sub.4 wherein A is an element in its divalent form and B is an element in its trivalent form. There is also known the case that A stands for a trivalent cation and B for a divalent cation. There come into consideration as divalent elements especially the elements Co, Ni, Cu, Zn, Cd, Mg, Mn, Fe and as trivalent metal ions Al, Cr, Ga, In, La, V.
In practice this multitude of elements also means a considerable breadth in variation of the colored body, especially in the coloristic pigment properties. This means that that inorganic bright colored pigments based on spinels represent pure chemical products only in the rarest case.
A known material is the cobalt-aluminum spinel, CoAl.sub.2 O.sub.4, which e.g. can be modified with Cr.sup.III .
This compound is also known under the trivial names cobalt blue, Leyden blue, Kings' blue and Thenard's blue. In ceramics they find use as blue coloring components and in the synthetic resin industry they are used as pigments for the production of light fast and weather resistant colors.
The customary processes for the production of cobalt pigments having a spinel structure are described in the appropriate literature. Thereby, e.g. cobalt oxide and aluminum oxide in powder form are vigorously intermixed and calcined at a temperature of around 1000.degree. C. for several hours in crucibles. As aids to the reaction there are frequently utilized so-called mineralizers, such as e.g., NaCl, CaCl.sub.2, KCl and CaSO.sub.4. Calcined products produced in such manner then are ground to pigment fineness. In enlargement these pigments show grain forms which resemble basalt splinters.
An alternative precipitation process with subsequent calcination for the production of cobalt blue is described in German OS No. 2840870. The covering power of this pigment, however, is very poor because of its small particle size, there are obtained only transparent blue tones.
However, for many uses it is necessary for pigments to have higher covering power and greater spreadability. Therefore it was the problem of the present invention to provide an inorganic pigment, especially a cobalt containing inorganic pigment, having a spinel structure which has a high covering power and a great spreadability. Besides there should be developed a process for the production of such pigments.